Method for producing coated die castings



A ril 2, 1963 A. F. BAUER 3,083,424

METHOD FOR PRODUCING COATED DIE CASTINGS Filed May 7, 1959 2Sheets-Sheet 1 IL VV INVENTOR Alfred F. Bauer ATTORN EY April 2, 1963 A.F. BAUER 3,083,424

METHOD FOR PRODUCING COATED DIE CASTINGS Filed May '7, 1959 2Sheets-Sheet 2 Fig. 4.

IN V EN TOR. Alfred E Bauer BY @N i J Q ire Alfred Ferdinand Bauer,Toledo, (Phi-o, assignor to l latlonal Lead Company, New York, Nil, acorporation of New .t'ersey Filed May 7, 1959, filer. No. 311,611 illGalina. (Cl. 22-293) This invention relates to a method for producingcoated die castings where the coating constitutes a protective or othersurface for at least a part of the die casting, and is particularlydirected to the production of a die casting having a surface coating orlayer which is strongly and permanently adhered to the body of the diecast material and is harder or more resistant to Wear or corrosion, orhas other required or desired characteristics which the body of the diecast material lacks.

As used herein, and in the appended claims, the word coating means arelatively thick and substantial body of a surfacing material which ispermanently mechanically locked to a die casting and distinguishes froma transitory or temporary coating such as would result from the use of adie lubricant or the like.

It has long een recognized that aluminum has numerous advantages overcast iron as a material for use in the pro duction of internalcombustion engine blocks, not the least of such advantages being its lowdensity, its high thermal conductivity, and its inertness or corrosionresistance with respect to cooling water. Aluminum is more expensivethan is gray iron, so that its use in an engine block can be justifiedeconomically only if sufficient savings are achieved in processing. tpresent, sufiicient processing savings can be achieved only when analuminum engine block is produced by a die casting technique. Before analuminum internal combustion engine block can be produced commerciallyby die casting, it is necessary to provide, among other things, awear-resistant cylinder bore surface which can be formed to a highdegree of accuracy Without expensive machining operations. The method ofthe instant invention for applying a protective or other surface to adie casting is admirably suited for providing a wear-resistant cylinderbore in a die cast aluminum internal combustion engine block.

Methods that have previously been suggested for providing awear-resistant cylinder bore surface have been found to be commerciallyunacceptable for one reason or another. For example, chrome plating canbe used to provide the required wear-resistance, but is anunrealistically expensive and difiicult expedient. Similarly,spray-coating techniques can be used to provide a wearresistant boresurface, but spray coating techniques require special precautions toachieve satisfactory bonding of the sprayed metal to an aluminum bore,and produce coatin s which are initially uneven and rough, so thatextensive machining is required. This, in turn, requires thespray-application of excessive amounts of a hard metal by comparisonwith the thin lining which remains after machining.

The instant invention is based upon the discovery of what has beendenominated a transplant method which can be used to provide a hard,wear-resistant, cylinder bore surface in an aluminum die cast enginecylinder sleeve, as well as in other castings of aluminum and othermetals. According to the method, such wear-resistant surface can be madewith such a high degree of accuracy that only a simple honing operationis required after the die casting operation. The casting as it is formedin practicing the preferred embodiment of the method requires no draftin the interior surface of the cylinder, and the interior surface is assmooth as the core over which it is formed.

The invention is also especially useful in the production 35,683,424Patented Apr. 2, 1963 of aluminum wheels for automotive vehicles. At thepresent time, work is being done to adapt a cast or forged aluminumwheel to this service because of the superior heat conductive qualitiesof aluminum alloys and because of their lighter weight. Since thealuminum is entirely unsuited for use as a braking surface, the presentproposals call for the use of a liner made from cast iron or a similarmaterial and bonded to the interior of the aluminum wheel, the linerfurnishing the braking surface against which the force of the brakingshoes is taken. The liners at the present time are quite thick and,because of their low heat-conductivity, operate at much highertemperatures at the braking surface than is desirable in spite of theability of the surrounding aluminum to dissipate quite rapidly the heatabsorbed at its interface with the liner. Since the physical propertiesof aluminum degenerate rapidly after its temperature reaches about 406F.,. the interface must not be too drastically heated, and thisrestriction has limited the application of aluminum Wheels to vehiclesin which the braking requirements are not too severe rather than for themost demanding service as was originally intended. It has been foundthat, by transplanting a liner layer to the interior of the Wheel, athin and still satisfactory braking surface may be provided and onewhich requires Virtually no machining after it is formed. The coatingformed on the casting by the method of the present invention may begiven any suitable thickness as will become apparent hereinafter, but ingeneral is much thinner than the liners previously proposed for knownaluminum wheels. Because the braking surface material is thin, itstemperature of operation will be much lower since the heat therefromwill be dissipated more rapidly into and from the aluminum body. Loweredoperating temperatures mean lower thermal expansion of the wheels With acorresponding reduction in the liability of brake fading, and increasedlife for the brake linings.

It has also been found that the method of the invention has utilityother than that in the automotive field. For

example, the interior of a die cast aluminum cooking utensil can be maderesistant to the corrosive action of certain foods by transplanting astainless steel surface thereon, or a die cast aluminum sole plate foran electric iron of either the steamor dry-type can be made stainproofby transplanting upon it a stainless steel surface. Aluminum bearingsmay have a bronze or other surface transplanted thereonto to providebetter bearing properties. Outer applications for the method of theinvention will be apparent to one skilled in the art from the followingdetailed discussion of the method.

it is, therefore, an object of the invention to provide an improvedmethod for producing a die casting having a coated surface.

It is a further object of the invention to provide a method forproducing such a die casting wherein the surface coating produced isextremely smooth and is of such accuracy that, at most, only a simplehoning operation is required for finishing.

It is still another object of the invention to provide a method forproducing a die cast cylinder sleeve for an internal combustion enginewhich is free fromtaper from end to end in its original state andrequires, therefore, no additional machining beyond a simple honingoperation.

It is still another object of the invention to provide a method forproducing a die cast lightweight wheel for an automotive vehicle havinga relatively thin wear-resistant coating exposed on its interior.

It is still another object of the invention to provide a method forapplying a coating to a portion of a die assembly and then transplantingthis coating onto an article produced in the die assem ly by a diecasting technique. Other objects and advantages will be apparent fromthe following detailed description, and from the attached drawings, inwhich-- FIG. 1 is a sectional view of a die casting die assembly whichcan be used to produce a typical die casting by the method ofthe-invention; a

FIG. 2 is an enlarged fragmentary plan view of a metalized surface of aportion of the die assembly of FIG. 1 prior to the injection of themolten metal thereinto;

FIG. 3 is a partially sectioned perspective view of a casting producedin the die assembly of 1 1G. 1; and

FIG. 4 is an enlarged fragmentary photograph showing the cross sectionalview of a typical casting produced by the method of the invention.

In the following specification, a specific example of an articleproduced by my new method, as well as a specific example of the methoditself will. first be described. Thereafter, the specification willinclude a description of various modifications and changes that may bemade to produce other and different articles 'of manufacture.

Referring now in more detail to the drawings, and in particular to FIG.1, a die cast aluminum cylinder sleeve for an internal combustion enginecan be produced in a die assembly which includes an ejector die 10, acover die 12, a-separate core member 14, and a shot sleeve 16.

- The core member 14 has a body portion 18 which fits into a socket 20provided therefor in the ejector die 10, there by accurately locatingthe core member concentric with a cylindrical cover die impression 22.Means for retaining the body portion of the core member within itsmating sockethas not been shown since this may be in the form of any ofseveral known mechanisms which are conventionally used by those skilledin the art. An annular die cavity 24 is thus formed between the coremember 14 and a generally cylindrical surface of the impression 22.

Asa; first step in producing the die cast cylinder sleeve, 3. coatinglayer 26 is applied to a cylindrical impression portion 28 of the core14. In contrast to the usual die casting techniques, the impressionportion 28 of the'core does not need to be tapered wherepracticing themethod of the invention in its preferred'embodiment. The coating layer26 may conveniently be applied to the impression portion 28 of the core14 by using a conventional metalizing gun to which a 420 stainless steel1 wire is supplied. Within the gun, the wire is fed into anoXy-acetylene flame where it is melted, and molten alloy is carried fromthe gun'by a blast of compressed air and, onto the impression portion28. It has been found to be preferable to roughen the impression surface'23 slightly, for example to-a smoothness of 20 to 30 RA LS? by lightsandblasting before application of the coating layer 26. Suchsandblasting promotes the proper adherence of the coating material tothe core portion. The thickness of the coating 26'should be from about0.015 inch to about 0.025 inch. The exterior of the coating 26 has anirregularly pitted and undercut surface exposed to the die cavity, as

can be seen readily in FIG. 2, which shows 'a portionof the exposedsurface ofsuch a coating, produced as described, by metalizing, enlargedapproximately eight times.

The application of the coating material by the conventional metalizinggun technique is particularly advantageous for the production ofcylinder sleeves for internal combustion engines for the reason that atleast portionsof' the coating material as applied are iron oxide, whichoxide portions provide hard and wear-resistant parts in the exposed areaof the liner.

Molten aluminum alloy of a selected type, under a pressure of from 4000to 10,000 pounds per square inch,

, ings.

is then forced into the mold cavity 24 (FIG. 1) to completely fill thedie cavity and to enter into all of the pits and undercuts on theerr-posed surface of the coating 26. As the molten metal solidifies, itshrinks in and around the coating'z and is interlocked mechanicallytherewith forming a bond of tremendous strength. Within a few secondsafter completion of injection of the aluminum, the two die halves areseparated, and the casting 30 and the core member 14 are ejected as aunit.

' In the preferred, although not essential, practice of the presentinvention, the casting is permitted to cool while still in place on thecore. in most instances, the core is made of a material having a lowercoefficient of thermal expansion than the casting, for example, steel.If the two parts are cooled together, the aluminum casting is restrainedby the core from shrinking to the full extent that it would shrink if itwere not supported in its interior. The aluminum is, in effect,stretched by the cooling process beyond its elastic limit and takes apermanent set at a diameter determined by the diameter of the core. Bypre-heating the core to a known temperature, and cooling the partstogetherto a known temperature, very accurate castings are obtained. Thevariation in diameter is within 2.001 inch in a 4 inch diameter sleeve.

The most convenient way of separating the core member 14 from the coatedcasting is by differential expansion of the parts. The aluminum castingwith the strongly bonded coating may be heated quickly while the coreremains cool and the expansion of the casting will allow the parts to beseparated very easily. A significant advantage of this method is thatthe core may be formed without taper so that the cylinder producedrequires no subsequent machining to produce a cylinder sleeve having auniform diameter from top to bottom.

It has been found possibl by this method, to produce a cylinder sleevehaving an interior surface coating, as die cast, smooth to 30 R.M.S. Thecompleted aluminum die castingfil with the ferrous alloy coating on theinterior surface thereof is shown in FIG. 3, with a portion broken awayto show details of the construction. A cross-sectional view of such acasting, enlarged approximately fourteen times, constitutesFlG. 4- ofthe drawln FIG. 4, the die cast body of the sleeve is designated 3i andthe coating is designated 25. The interlocking of the material of thecoating and the body of the sleevecan readily be seen. I

it will be appreciated that the cylinder sleeve 38 is illustrative ofarticles which can be produced by the method of the invention, and that,in the sleeve, a coating is transplanted from a die portion to the castsleeve. It will be understood that, in the cylinder sleeve, thetransplanted coating serves a principal function of providingwearresis'tance and oil retention, but that coatings which provide otherdesired properties may be applied either to similar die castings or todie castings of othervaried shapes by this method, and that variouschanges can be made from the specific details set forth above. Forexample, coatings either substantially thicker or significantly thinnerthan the 0.015 inch to 0.025 inch range may be preferred for specificapplications. Also, the coating may be applied to a die member, whichcan be either a fixed part of a die or a removable part, and variousways other than by metalizing. The die member can be dipped into avessel containing a suspension of a desired coating material in asuitable carrier which is capable of causing the particles from whichthe coating is to be formed to adhere to the die sufficiently that theywill not be washed away by metal entering the die during the castingoperation. The carrier for the particles can be of such a nature that,upon its evaporation, the exposed surface of the coating will be roughand will contain pits and undercuts similar to asprayed coating. Asimilar type of a coating material can be applied to the die by,brushing or painting, or a machining operation might be used to providethe preferred rough surface on an originally smooth coating.

It is usually preferred, for economic reasons, to apply a coating, inpracticing the method of the invention, to a die part which is separablefrom the remainder of the die. By operating in this manner, a number ofidentical die members can be coated as a preliminary operation, and thenused as required so as not to slow down the machine cycle. If preferred,however, for any reason, the coating can be applied to a die or a diepart which is mounted in a die casting machine.

The cylinder sleeve 3%, produced as described above, is a part producedaccording to the method of the invention wherein a smooth polishedsurface is desired. The method of the invention is equally effective toproduce other parts where a surface that is other smooth, for examplehaving a desired or required pattern, is preferred. in such case, it isnecessary only to provide the desired or required pattern on the surfaceof the die part, so that the coating, when applied to the die part,follows the contour thereof, and, upon transplanting, irnparts suchcontour to the exposed surface of the casting.

A metalized or metal-sprayed coating, due to the manor of application,contains minute voids. For most applications, it is preferred that thecoating be or" sufficient thickness and density that the voids do notextend through the coating layer. This is to prevent the passage ofmolten metal, dorin casting, through the voids to the surface which willbe exposed. The density of the coating may be altered according to thetechnique employed in applying it to the die. If the coating is appliedby spraying, the density thereof may be varied according to the distancemaintained between the metalizing gun and the die; a technique known tothe rnetalizing art. If the coating is applied by dipping or painting,its density may be varied according to the composition of the mixture.

Castings made by the method of the present invention may readily begiven continuous coatings of varying depths and of varying properties.For example, a cylinder sleeve for an internal combustion engine issubject to unequal wear, unequal thermal stress, and unequal exposure tocorrosive deposits from end to end. The area forming part of thecombustion chamber is subject to the highest heat and to the highestwear, and must exhibit the greatest resistance to corrosive depositsresulting from combustion. immediately below this, in the area swept bythe piston, the temperatures encountered are progressi ely lower and,below the line of upper reversal of the piston rings, the wearencountered is much less. At the lower end of the cylinder only limitedwear takes place, and this is largely concentrated at the line of lowerreversal of the piston rings at their bottom dead center position. Mostadvantageously, then, the coating that is formed in the casting shouldbe chosen for heat and corrosion resistance at the top, for extremewearresistance at the lines of piston ring reversal, and for oilretention and somewhat more moderate resistance to wear in theintermediate areas. A continuous coating composed of annular bands ofdifferent materials, and of different thicknesses can be deposited onthe core 14 for the purposes outlined. Stainless steels, ceramic andcermet materials may be used for the portions of the sleeve meeting themost severe conditions, a molybdenum containing coating for thoseportions subject to the greatest wear, and much less expensive steelcoatings for the balance of the sleeve.

The method or" this invention can best be practiced by the process ofpressure die casting where the pressure exerted on the molten metal inthe die cavity is effective to force it into mechanical interlockingrelationship with the irregular surface of the coating. Such mechanicalinterlocking must produce, upon solidification of the die cast metal, astronger bond between the coating and the metal than that existingbetween the coating and the wall of the die. Then, upon solidificationof the metal, the

solidified casting and the mechanically interlocked coating are removedfrom the die. When, as is preferred, the surface of the coating that isexposed in the die cavity is pitted and undercut, the mechanical bond issufliciently strong that, in spite of significant differences in thermalexpansion properties between the two metals, the casting and coating donot separate even at elevated temperatures. For example, such diecastings of aluminum with ferrous metal coatings do not separate even atthe temperatures encountered in the combustion chamber of an internalcombustion engine.

In its essential details, the method of the invention is for producing adie casting having a coated surface. Such method comprises applying acoating to a portion of a die constituting at least a part of a wall ofa die cavity, which coating has an irregular surface exposed to thecavity, injecting molten metal into the die cavity and into mechanicalinterlocking relationship with the coating, the mechanical interlockingrelationship producing upon solidification a stronger bond between thecoating and the injected metal than the coating has to the die Wall,solidifying the metal in the die cavity, and removing the solidifiedcasting and the coating from the die cavity.

The method of this invention can also be employed to transplant acoating from any other portion of the die cavity to the complementaryportion of a pressure die casting. The examples given are for thepurpose of illustration only and are not to be considered as limitingthe scope of the followin claims.

What 1 claim is:

l. The method of producing a die casting having a taper-free cylindricalinterior surface of a metal harder and of higher melting point than thebody of the casting which comprises, initially forming a coating of ahigher melting point harder metal on a smooth cylindrical coreconstituting a part of a wall of a die cavity by depositing a coating ofsuch metal directly on said core, said coating having a rough and pittedsurface exposed to said cavity, injecting molten metal of a meltingpoint lower than said coating metal into said die cavity under pressuresuilicient to force said molten metal into intimatemechanical'interlocking relationship with said coating, said mechanicalinterlocking relationship producing upon solidification a stronger bondbetween the coating and the iniected metal than the coating has to saidsmooth cylindrical core, solidifying the injected metal in the diecavity whereby the injected metal shrinks around said coated core,removing the solidified casting and coated core from the die cavity,cooling said casting and core together to a temperature sufiiciently lowto cause a permanent deformation of said casting and coating, reheatingat least said casting whereby ditlerential expansion of said core andcasting causes radial separation thereof, and separating said core fromsaid casting and interlocked coating. I

2. The method of producing a die casting having a cylindrical interiorsurface of a material harder and of higher melting point than the bodyof the casting which comprises, initially forming a coating of a highermelting point harder material on a smooth cylindrical core constitutinga part of a wall of a die cavity by depositing a coating of such metaldirectly on said core, said coating having a rough and pitted surfaceexposed to said ca ity, injectin molten metal of a melting point lowerthan said coating material into said die cavity under pressure suficientto force said molten metal into intimate mechanical interlockingrelationship with said coating, said mechanical interlockingrelationship producing upon solidification a stronger bond between thecoating and the injected metal than the coating has to said smoothcylindrical core, solidifying the injected metal in the die cavitywhereby the injected metal shrinks around said coated core, removing thesolidified casting and coated core from the die cavity, cooling saidcasting and core together to a temperature sutficiently low to cause aperaccuses m-anent deformation of said casting and coating, re-heatingat least said casting whereby differential expansion ofsaid core andcasting causes radial separation thereof, and separating said core fromsaid casting and interlocked coating.

3. The method of producing a die casting having a cylindrical interiorsurf-ace of a material harder and of higher melting pointthan the bodyof the casting which comprises, initially depositing a coating of ahigher Incling point harder material on a smooth cylindrical coreconstituting a part of a wall of asdie cavity, said coating having arough and pitted surface exposed to said cavity, injecting molten metalof a melting point lower than said coating material into said die cavityunder pressure sufiicient to force said molten metal into intimatemechanical interlocking relationship with said coating, said mechanicalinterlocking relationship producing upon solidification a stronger bondbetween the coating and the injected metal than the coating has to saidsmooth cylindrical core, solidifying the injected metal in the diecavity whereby the injected metal shrinks around said coated core,removing the solidified casting and coated core from the die cavity, andseparating said core from said casting and interlocked coating.

4. The method of producing a pressure casting having an interior surfaceof a material, harder and of higher melting point than the body of thecasting which comprises, initially depositing a coating of a highermelting point harder material on a smooth core constituting a part of awall of a mold cavity, said coating having a rough-and pitted surfaceexposed to said'cavity, introducing under pressure molten metal of amelting point lower than said coating material into said mold cavity,establishing on said introduced metal in said cavity a pressuresufficient to force said introduced metal into intimate mechanicalinterlocking relationship with said coating, said mechanicalinterlocking relationship producing upon solidification a stronger bondbetween the coating and the introduced metal than said coating has tosaid core, solidifying said casting around said coated core, reheatingat least said casting whereby differential expansion of said core andcasting causes separation thereof,

and separating said core from said casting and interlocked coating.

5. The method of producing a die casting haying a cylindrical interiorsurface of a metalharder and of higher melting point than the body ofthe casting which comprises, initially depositing a coating of a highermelting point harder metal on a smooth cylindrical core con-stituting apart of a wall of a die cavity, said coating having a rough and pittedsurface exposed to said cavity, injecting molten metal of a meltingpoint lower than said coating metal into. said die cavity under pressuresufiicient to force said molten metal into intimate mechanicalinterlocking relationship with said coating, said mechanicalinterlocking relationship'producing upon solidification a stronger bondbetween the coating and the injected metal than the coating has to saidsmooth cylindrical core, solidifying the iniected metal in the diecavity whereby the injected metal shrinks around said coated core,establishing a lower temperature in said core than in said castinwhereby differential expansion of said core and casting causes radialseparation thereof, and separating said core from said casting andinterlocked coating. 6. The method of producing a die casting having acoated inner cylindrical surface which comprises, initially depositing acoating on a cylindrical core portion of a die constituting at least apart of a wall of a die cavity, said coating having a rough and pittedsurface exposed to said cavity, injecting under pressure molten metalhaving a higher coefiicient of thermal expansion than said core intosaid die cavity and into mechanical interlocking relationship with saidcoating, said mechanical interlocking relationship producing uponsolidification a stronger bond between the coating and the injectedmetal than the coat ing has to said core portion, solidifying theinjected metal in the die cavity, removing the solidified casting andthe coated core portion from the die cavity, cooling the core andcasting together to a temperature sufiiciently low to cause permanentdeformation of said casting and coating and separating the solidifiedcasting and interlocked coating from the core portion.

7. The method of producing a die casting having a surface of a metal ofa higher melting point than the body of the casting which comprises,initially depositing a coating of a higher melting metal on a portion ofa die constituting at least a part of'a wall of a die cavity, saidcoating having a rough and pitted surface exposed to said cavity,injecting under pressure molten metal of a melting point lower than saidcoating metal into said die cavity, and into mechanical interlockingrelationship with said coating, said mechanical interlockingrelationship producing upon solidification a stronger bond between thecoating and the iniected metal than the coating has to said die wall,solidifying the metal in the die cavity, and removing the solidifiedcasting and the interlocked coating from the die cavity.

8. An article of manufacture produced by the method of claim 7. a

9. The method of producing a casting having a surface of a metal of ahigher melting point than the body of the casting which comprises,initially depositing a coating of a higher melting metal on a portion ofa mold constituting at least a part of a wall of a mold cavity, saidcoating having a rough and pitted surface exposed to said cavity,introducing under pressure molten metal of a melting point lower thansaid coating metal into said mold cavity, forcing said molten metal intomechanical interlocking relationship with the exposed surface of saidcoating, said mechanical interlocking relationship producing uponsolidification a stronger bond between the coating and the introducedmetal than the coating has to said mold wall, solidifying the metal inthe mold cavity, and removing the solidified casting and the interlockedcoating from the mold cavity.

it). The method of producing a casting having a coated surface whichcomprises, initially depositing a coating on a portion of a dieconstituting atleast a part of a wall of a die cavity, said coatinghaving an irregular surface exposed to said cavity, introducing underpressure molten metal into said die cavity, forcing said molten metalinto mechanical interlocking relationship with said coating,said'mechanical interlocking relationship producing upon solidificationof said introduced metala stronger bond between the coating and themetal than the coating has to said die wall, solidifying the metal inthe die cavity, and

, removing the solidified casting and the interlocked coating from thedie cavity.

References 'Citcd in the file of this patent UNITED STATES PATENTS JeponSept. 24, 1957 OTHER narnanncas Practical Considerationsin Die CastingDesign, New Jersey Zinc. 00., copyright 1948, printed by MarhridgePrinting Co., New York 14, N.Y.; p. 152 relied on.

Die Casting, by H. H. Doehlencopyright 1951, first edition, McGraw-HillBook Co., New York; page relied on.

1. THE METHOD OF PRODUCING A DIE CASTING HAVING A TAPER-FREE CYLINDRICALINTERIOR SURFACE OF A METAL HARDER AND OF HIGHER MELTING POINT THAN THEBODY OF THE CASTING